Colonography, the use of electronic imaging technologies such as computed tomography (CT) to generate images of a patient's colon for purposes of colorectal cancer screening, is generally known. By way of example, these technologies are disclosed in the Johnson et al. U.S. Pat. Nos. 6,928,314 and 7,035,681, the Zalis U.S. Pat. No. 6,947,784, the Vining U.S. Pat. Nos. 6,909,913 and 7,149,564, and PCT publication no. WO 2007/030132, all of which are incorporated herein by reference. Briefly, this methodology involves obtaining a series of CT images of adjacent portions or slices of the colon. A radiologist then studies each of the images to identify any pre-cancerous polyps. Alternatively, a computer can effectively create a simulated intraluminal flight through the colon (this is also known as virtual colonoscopy). Colonography has been demonstrated to be a highly efficacious approach for detecting colorectal polyps.
When viewing CT images on a computer screen, the images typically have an intrinsic “dynamic range” of 4096 distinct gray levels (that is, one of 4096 different intensity values can be recorded at every pixel). However, a conventional computer monitor can typically only display 256 distinct gray levels. There are thus many possible ways to convert the measured data for viewing. For example, one can display the 256 lowest gray levels, with everything above this being displayed as pure white; one could display the brightest 256 levels, with everything darker than this being displayed as pure black; or one could evenly apportion the 4096 possible levels among the 256 available levels by dividing by 16, giving the most faithful overall rendition but losing much of the possible discriminating ability present in the data. In practice, different such mappings (known as window/level settings) are used for different purposes. For example, for optimal viewing of the lung, which is very dark, discrimination in bright parts of the image is of limited value. Thus, all bright pixels can be set to white and the available gray levels reserved for use in the appropriate brightness range. Radiologists have defined many such window/level settings that are known and in standard use to highlight different parts of the anatomy.
In a 2D axial review, radiologists often scan the colon data twice on a computer screen. The colon can first be viewed with a “lung” window/level setting that clearly shows protrusions into the lumen that are indicative of polyps. When such a protrusion is found, the reader may switch to a “tissue” window to determine if the object consists of soft tissue or fat. A second pass can be made in the reverse direction at the “tissue” window/level setting to look for areas of wall thickening that are indicative of flat lesions.